60-NITINOL is a superelastic ordered intermetallic compound that is well suited for shock-resistant aerospace bearing applications and formed from nickel (Ni) and titanium (Ti). 60-NITINOL was discovered in the 1960s, but was not commercialized because it was difficult to machine with conventional technology at that time, partly because of the issue of residual stresses and quench cracking. However, there is renewed interest in the material within the aerospace community since it can be machined using techniques that have been developed since its discovery and due to its unique combination of physical properties.
60-NITINOL is corrosion-proof, electrically conductive, non-magnetic, and benign in the presence of conventional lubricants. 60-NITINOL, paradoxically, is also very hard, yet highly resistant to damage from shock loads. The high hardness of this material, which is critical to its use in mechanical components such as bearings, is achieved through heat treatment.
However, the high temperatures required to heat treat the material to maximum hardness have been found to cause thermal distortion and, in some cases, quench cracking, which can render an article unusable after an investment of many hours of machining time. A partial phase diagram 100 for binary Ni—Ti is shown in FIG. 1. FIG. 1 shows the phase structure of Ni—Ti alloys as a function of temperature and composition, where the top horizontal axis indicates the amount of Ti by weight percent and the bottom horizontal axis indicates the amount of Ti by atomic percent. The Naval Ordnance Laboratory initially identified the potential uses of compositions consisting of 55% and 60% Ni by weight, with the balance being Ti, and named these compositions 55-NITINOL and 60-NITINOL, respectively. However, the phase diagram indicates an entire range of compositions from approximately 55-61% Ni by weight (50-57 atomic percent Ni). The hardness of the material increases with increasing Ni content within this composition range (i.e., the γ-phase field in FIG. 1), which requires concomitantly higher solution treatment temperatures.
As seen in the Ni—Ti phase diagram, austenitic NiTi (the phase labeled γ at the center of FIG. 1) is formed when NiTi is heated above the solvus line (at approximately 1050° C. for 60-NITINOL) and then immediately quenched to room temperature. This process is known as a solution treatment. The solution treated material can then be reheated to an intermediate temperature in a process known as aging where small precipitates coalesce and grow.
These precipitates increase the hardness of the material. However, the thermal stresses created by quenching from above 1000° C. can cause the material to warp. If the stress within a component encounters a stress riser, such as a sharp radius or near surface defect (e.g., an inclusion, a pore, a machining defect, or another surface disparity), the component can fracture. 60-NITINOL has many technical advantages, but challenges arise in heat treating the material to sufficient hardness for aerospace component applications without producing residual stresses that result in cracked or distorted parts. Accordingly, an improved material that maintains hardness, but mitigates against thermal distortion and quench cracking, may be beneficial.